In general, a conventional ultrapure water manufacturing system is composed of three stages of apparatuses, such as a pretreatment apparatus, a primary pure water manufacturing apparatus, and a secondary pure water manufacturing apparatus. However, in recent years, the level of required water quality has become higher along with miniaturization of semiconductors and, therefore, systems to obtain higher-purity water quality by further disposing a tertiary pure water manufacturing apparatus downstream from the secondary pure water manufacturing apparatus have been adopted in many cases.
In the pretreatment apparatus of the ultrapure water manufacturing system, a pretreatment through filtration of raw water, coagulation sedimentation, microfiltration membrane, or the like is performed and, thereby, suspended substances are removed mainly. The number of fine particles in the water results in usually 103 particles/mL or less by this pretreatment.
The primary pure water manufacturing apparatus is provided with a reverse osmosis (RO) membrane separation apparatus, a deaeration apparatus, a regenerative ion exchange apparatus (mixed bed, 4-bed 5-tower, or the like), an electric deionization apparatus, an oxidation apparatus, e.g., an ultraviolet (UV) irradiation oxidation apparatus, and the like, and removes most of electrolytes, fine particles, viable bacteria, and the like in the pretreated water. The primary pure water manufacturing apparatus is composed of, for example, two RO membrane separation apparatuses and a mixed bed ion exchange apparatus or an ion exchange pure water apparatus and an RO membrane separation apparatus.
The secondary pure water manufacturing apparatus is composed of a feed water pump, a heat exchanger, an ultraviolet irradiation apparatus, e.g., a low-pressure ultraviolet oxidation apparatus or a pasteurizer, a non-regenerative mixed bed ion exchange apparatus or electric deionization apparatus, and a membrane filtration apparatus, e.g., an ultrafiltration (UF) membrane separation apparatus or a microfiltration (MF) membrane separation apparatus. However, in some cases, a deaeration apparatus, e.g., a membrane deaeration apparatus or a vacuum deaeration apparatus, an RO membrane separation apparatus, and a desalting apparatus, e.g., an electric desalting deionization apparatus, may be disposed. The secondary pure water manufacturing apparatus reduces a TOC load on the tertiary pure water manufacturing apparatus downstream therefrom so as to reduce the TOC components in the tertiary pure water (ultrapure water) to the limit by applying a low-pressure ultraviolet oxidation apparatus, disposing a mixed bed ion exchange apparatus downstream therefrom and, thereby, oxidizing and decomposing organic materials (TOC components) in the water with ultraviolet rays and removing the oxidative decomposition products through ion exchange.
The tertiary pure water manufacturing apparatus has the same apparatus configuration as the configuration of the secondary pure water manufacturing apparatus and, thereby, further purify the secondary pure water to produce high-purity ultrapure water.
Main reaction mechanisms and purposes of the individual units of the secondary pure water manufacturing apparatus and the tertiary pure water manufacturing apparatus are as described below.
i) Low-pressure ultraviolet oxidation apparatus; Residual
TOC coming from the upstream stage is oxidized and decomposed to carbon dioxide or organic acids, e.g., carboxylic acid, by ultraviolet rays with a main wavelength of 185 nm.
ii) Mixed bed ion exchange apparatus; Residual carbonate ions, organic acids, and anionic substances, which result from decomposition through ultraviolet oxidation, and metal ions and cationic substances, which come from the upstream stage, are removed through ion exchange.iii) Deaeration apparatus; Entrained dissolved gases, e.g., DO (dissolved oxygen), are removed.iv) UF membrane separation apparatus; Fine particles are removed.
FIG. 2 is a flow diagram showing the typical apparatus configurations of conventional secondary pure water manufacturing apparatus 20 and tertiary pure water manufacturing apparatus 30. In the drawing, RO, UVox, MDI, Deaeration, and UF indicate the following.
RO; Reverse osmosis membrane separation apparatus
UVox; Ultraviolet oxidation apparatus
MDI; Mixed bed ion exchange apparatus
Deaeration; Deaeration apparatus
OF; Ultrafiltration membrane separation apparatus
Regarding an ultrapure water manufacturing system shown in FIG. 2, in the secondary pure water manufacturing apparatus 20, primary pure water passed through a tank 1 is treated with a reverse osmosis membrane separation apparatus 2, an ultraviolet oxidation apparatus 3, and a mixed bed ion exchange apparatus 4 sequentially, so that secondary pure water is obtained. A part of the secondary pure water is recycled to the tank 1 and the remainder is fed to the tertiary pure water manufacturing apparatus 30. The secondary pure water introduced into the tertiary pure water manufacturing apparatus 30 is passed through a tank 5 and is treated with an ultraviolet oxidation apparatus 6, a mixed bed ion exchange apparatus 7, a deaeration apparatus 8, and an ultrafiltration membrane separation apparatus 9 sequentially, so that tertiary pure water (ultrapure water) is obtained. A required amount of the resulting ultrapure water is fed to a use point and excess water is returned to the tank 5.
In this regard, in general, the “pure water” here refers to high-purity water specified on a use basis, and is specified here as described below for the sake of convenience. However, in some cases, the specified range is not conformed depending on the quality of raw water or the system configuration.
(A) Primary Pure Water
Electrical resistivity; 10 MΩ·cm or more
TOC; 5 to 50 μg/L
(B) Secondary Pure Water
Electrical resistivity; 18 MΩ·cm or more
(Metal ion concentration: 5 ng/L or less, residual ion concentration: 10 ng/L or less)
The number of fine particles; 5 or less of fine particles of 0.1 μm or more in 1 mL
TOC; 1 to 10 μg/L
(C) Tertiary Pure Water
TOC; 0.1 to 5 μg/L
The number of fine particles; 5 or less of fine particles of 0.1 μm or more in 1 mL
By the way, the mechanism of oxidative decomposition of the TOC component in the ultraviolet oxidation apparatus is to oxidize and decompose water so as to generate OH radicals and oxidize and decompose the TOC component with the resulting OH radicals. Usually, in the ultraviolet oxidation apparatus of the secondary pure water manufacturing apparatus and also in the ultraviolet oxidation apparatus of the tertiary pure water manufacturing apparatus, the amount of ultraviolet irradiation is specified to be excess irradiation in such a way that TOC in the water can be oxidized and decomposed sufficiently. However, regarding the ultraviolet oxidation apparatus in which the amount of ultraviolet irradiation is large, as described above, in the case where the TOC concentration in the water to be treated is low, OH radicals generated through decomposition of the water become excess. Therefore, excess radicals associate, so as to become hydrogen peroxide. The resulting hydrogen peroxide is decomposed when coming into contact with an ion exchange resin of the downstream mixed bed ion exchange apparatus. At that time, the ion exchange resin is degraded, and fresh TOC derived from the ion exchange resin is generated because of decomposition of the ion exchange resin so as to cause degradation of the quality of the obtained ultrapure water. Furthermore, hydrogen peroxide, which still remains after the water is passed through the mixed bed ion exchange apparatus, degrades the deaeration apparatus and the UF membrane downstream from the mixed bed ion exchange apparatus.
In addition, if hydrogen peroxide is decomposed in the ion exchange apparatus, and furthermore, the deaeration apparatus and the UF membrane separation apparatus downstream therefrom, the following reaction occurs to generate oxygen, so that DO in the water increases.2H2O2→2H2O+O2 
Then, in order to solve the above-described problems resulting from hydrogen peroxide, it has been proposed that an anion exchange tower filled with an anion exchange resin or an adsorption tower filled with a carbon based adsorbing agent is disposed between the ultraviolet oxidation apparatus and the mixed bed ion exchange apparatus, hydrogen peroxide generated in the ultraviolet oxidation apparatus is removed at the stage upstream from the mixed bed ion exchange apparatus and, thereafter, the water is passed through the mixed bed ion exchange apparatus.
The problems related to degradation of the ion exchange resin of the mixed bed ion exchange apparatus and degradation of the deaeration apparatus and the like downstream therefrom, which result from hydrogen peroxide, are solved by disposing the anion exchange tower or the adsorption tower and removing hydrogen peroxide at the stage upstream from the mixed bed ion exchange apparatus. However, regarding the conventional pure water manufacturing apparatuses, sufficient consideration has not been given to the generation of TOC in the anion exchange tower or the adsorption tower. Therefore, there is a problem in that ultrapure water having a low TOC concentration cannot be obtained by applicating them to the secondary pure water manufacturing apparatus.
That is, hydrogen peroxide comes into contact with the anion exchange resin and, thereby, hydrogen peroxide is decomposed and removed. However, along with this, a problem occurs in that TOC components derived from the resin is eluted because of degradation of the anion exchange resin.
Furthermore, regarding the carbon based adsorbing agent as well, there is a TOC elution problem.
Moreover, regarding decomposition of hydrogen peroxide with the anion exchange resin or the activated carbon, problem remains in that the decomposition rate is low, hydrogen peroxide is not removed sufficiently and, therefore, oxygen is generated at the downstream stage so as to increase DO.
Consequently, development of a technology to prevent an occurrence in itself of hydrogen peroxide in the ultraviolet oxidation apparatus is desirable rather than removal of hydrogen peroxide generated in the ultraviolet oxidation apparatus at the stage upstream from the mixed bed ion exchange apparatus.
In this regard, Patent Document 1 described below proposes a method in which water to be treated is brought into contact with a hydrogen peroxide decomposition catalyst including a carrier supporting platinum group metal nanoparticles having an average particle diameter of 1 to 50 nm to decompose hydrogen peroxide in the water, and describes that a hydrogen peroxide decomposition apparatus filled with the above-described hydrogen peroxide decomposition catalyst is disposed downstream from the ultraviolet oxidation apparatus. However, generation in itself of hydrogen peroxide in the ultraviolet oxidation apparatus is not prevented.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2007-185587